[Johnson] Transformer "cooking"

[Carl]

As a retired engineer I often cringe at ideas not taught in university or on 
the job but I rarely dismiss them outright as decades of R&D experience have 
shown me.

Since the transformer baking suggestions and successes have been around 
longer than those on this forum have been alive it deserves serious 
consideration.

If the turns ratio is a concern simply reverse the hook up. A hi-pot will 
tell how much leakage there is but not everybody has one. However 
constructing one to test up to 1000V or so is simple enough and there are 
plenty of old microwave ovens and vintage scopes around to get up to 5KV or 
so with some serious current limiting.
I have 2 testers, one 0-5KV commercial grade for $25 from a local surplus 
electronics company. The homebrew uses a sign transformer and hamfest HV 
caps along with an old rectifier assembly; this one is mostly used to test 
transmitting tubes.

With transformer and motor shops going the way of the dodo bird in many 
areas of the US doing as you suggest is not possible. Shops these days are 
seldom interested in a hobbyist job....we all know how cheap hams can be.

For a typical vintage ham amp having a transformer rewound is about 60% the 
cost of a new one, BUT you are still dealing with inefficient and obsolete 
laminations and a high impedance design suitable for tube rectifiers in a 
full wave circuit.

Carl
KM1H





----- Original Message ----- 
From: "Dennis Deaton" <wa6acc at verizon.net>
To: <johnson at mailman.qth.net>
Sent: Tuesday, January 07, 2014 2:24 PM
Subject: Re: [Johnson] Transformer "cooking"


On 01/06/2014 12:59 PM, Roy Morgan wrote:
> On Jan 6, 2014, at 3:14 PM, Carl <km1h at jeremy.mv.com> wrote:
>
>> I dont know if Measures used the method I described decades ago but there 
>> are methods that work.
>> ...
>> Short the secondary with an ampmeter and fuse in series. Place a Variac 
>> on the primary with another ampmeter and fuse. ...
>> Bring up the Variac very slowly while monitoring both meters. ...
>> It might take days or even weeks until it is running at even half its CCS 
>> KVA rating;
> I assume you mean the CURRENTS that would be seen if it were running 
> normally under full load.
>
>> consider the electric bill vs saving money on a new transformer.
>>
>> The last one I recovered was ... a 275# monster ...
>>
>> It took a week of cooking out in the garage, at up to around 75% of 
>> calculated KVA, ... followed by a dunking in a vat of transformer varnish 
>> after a hi pot at 13.5KV which was the limit of my tester.
>   Again, I assume you mean the primary and/or secondarily CURRENTS that 
> would be expected in normal operation.
>
> I’ll check my garage for a vat of transformer varnish. heheh
>
> I’m pretty sure I don’t have a variac the right size for a 275 pound 
> transformer, but:  How about using a lower voltage transformer such as a 
> monster filament transformer, to feed the patient being warmed?  I wonder 
> what sort of voltage the primary of your monster took during the warming 
> process.
>
> Thanks much for your post.  It gets added to my transformer notes file.
>
> Roy
>
> Roy Morgan
> RoyMorgan at alum.mit.edu
> K1LKY Since 1958 - Keep 'em Glowing!
>
>
>
>
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Hi guys,

I've been following this thread for a while. As a retired engineer I
cringe at the thought of applying any voltages to a wet transformer
(especially one that has a high turns-ratio like a plate transformer)
with even a variac. Somewhere in that stack of windings the voltage will
exceed the insulating qualities of the wet paper insulation and cause an
arc to the next set of windings or the core.

Probably the best way of handling this situation is to take the
transformer to a motor / transformer rewinding shop and have them bake
it in their curing oven. Most or all of the reputable shops have them
and they will do this, probably, for a nominal fee. After that, you can
have them dis-assemble the core from the windings and dip it in
insulating varnish and re-bake it. I know that sounds like a lot. But
you'll end up with a better-than-original transformer in the end.

Dennis Deaton, Redlands, CA
WA6ACC
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